Abstract : Semi-parametric methods are often used for the estimation of intervention effects on
correlated outcomes in cluster-randomized trials (CRTs). When outcomes are missing at random
(MAR), Inverse Probability Weighted (IPW) methods incorporating baseline covariates can be used
to deal with informative missingness. Also, augmented generalized estimating equations (AUG)
correct for imbalance in baseline covariates but need to be extended for MAR outcomes. However,
in the presence of interactions between treatment and baseline covariates, neither method alone
produces consistent estimates for the marginal treatment effect if the model for interaction is not
correctly specified.We propose an AUG-IPW estimator that weights by the inverse of the probability
of being a complete case and allows different outcome models in each intervention arm. This estimator
is doubly robust (DR), it gives correct estimates whether the missing data process or the outcome
model is correctly specified. We consider the problem of covariate interference which arises when the
outcome of an individual may depend on covariates of other individuals. When interfering covariates
are not modeled, the DR property prevents bias as long as covariate interference is not present
simultaneously for the outcome and the missingness. An R package is developed implementing the
proposed method. An extensive simulation study and an application to a CRT of HIV risk reduction-
intervention in South Africa illustrate the method.